1. Field of the Invention
The present invention relates to an electrophotographic photoconductor and an image forming apparatus. The image forming apparatus of the present invention can be favorably applied to copiers, facsimiles, laser printers, direct digital platemakers and the like.
2. Description of the Related Art
Although electrophotographic photoconductors for use in photocopiers, laser printers and the like were formerly dominated by inorganic photoconductors such as those including selenium, zinc oxide, cadmium sulfide, etc., organic photoconductors (OPCs), which are more advantageous than the inorganic photoconductors in terms of reduction in load on the global environment, cost reduction and high design freedom, are most popular at present. The organic photoconductors are currently used at a rate that is close to 100% of the total production volume of electrophotographic photoconductors. In response to the present-day protection of the global environment, the role of the organic photoconductors is required to change from that of consumable supplies (disposable products) to mechanical components.
In the past, various attempts have been made to impart high durability to organic photoconductors. At present, formation of a cross-linked resin film on a photoconductor surface (e.g., Japanese Patent Application Laid-Open (JP-A) No. 2000-66424) and formation of a sol-gel cured film on a photoconductor surface (e.g., JP-A No. 2000-171990) are deemed particularly promising. The former has an advantage in that the photoconductor surface does not easily allow breaks or cracks to be formed therein even when a charge transporting constituent is mixed in the film forming composition, and a reduction in production yield can be prevented. In particular, use of a radically polymerizable acrylic resin is advantageous because a photoconductor having high strength and excellent photosensitive properties is easily obtained. In these two methods utilizing cross-linked structure, a coating film is formed by a plurality of chemical bonds, and thus even when the coating film receives physical stress and part of the chemical bonds is cut, this does not soon lead to abrasion (wear) of the photoconductor surface.
In addition, the use of polymerization toners (spherical toners) is becoming increasingly common for the toners for development used in electrophotography for reasons of improved reduction in the load on the environment during toner production as well as being advantageous in terms of achieving high image quality.
These polymerization toners (spherical toner) are spherically shaped toners that are free of corners, and are produced by chemical production processes such as suspension polymerization, emulsion aggregation polymerization, ester elongation polymerization and dissolution suspension. The polymerization toners vary in shape depending on the production process, and the polymerization toner used in image forming apparatuses has a shape that is slightly distorted from being perfectly round. Typical characteristic values consist of an average circularity of 0.95 to 0.99, values of 110 to 140 for shape factors SF-1 and SF-2. Those having an average circularity of 1.0 and a value of 100 for the shape factors SF-1 and SF-2 are perfectly round.
Since polymerization toners have a uniform shape, the charge they possess can be relatively easily made uniform. In addition, wax and the like can be easily added into the polymerization toners. Thus, since there is hardly any overflow of the polymerization toners from latent electrostatic images, the polymerization toners have satisfactory developing ability, superior sharpness, superior resolution, superior contrast and satisfactory transfer efficiency. In addition, they also have numerous advantages such as oil-less transfer. On the other hand, toners of this type are known to be associated with difficulties in cleaning, and troubles such as fish-shaped filming on photoconductors as a result of an increase in the amount of an external additive in relation to the oil-less transfer. Accordingly, numerous countermeasures against the foregoing have been proposed in patent literatures and the like.
In general, in order to ensure adequate cleanability for the polymerization toners, it is desirable that the surface of the photoconductor have a low friction coefficient and that the friction coefficient be maintained during repeated use. For example, cleanability for a polymerization toner is known to be secured by coating the surface of a photoconductor with a solid lubricant such as zinc stearate (Nobuo Hyakutake, Akihisa Maruyama, Satoru Shigesaki, Sachie Okuyama: Japan Hardcopy Fall Meeting, 24-27, 2001).
However, when a solid lubricant such as zinc stearate is externally supplied to a highly durable electrophotographic photoconductor with the radically polymerizable acrylic cross-linked film, there is a problem in which the solid lubricant cannot be not fully accepted by the photoconductor surface. In many cases, photoconductors of this type have smooth surfaces. Therefore, the defect of acceptability is attributable to the smoothness of the photoconductor surface.
Meanwhile, JP-A No. 2007-121908 proposes use of a mixture of (A) a fatty acid metal salt and a lubricant powder material (B) (which is made of at least one lubricative material selected from melamine cyanurate, ethylene polytetrafluoride, molybdenum disulfate and a fatty acid amide) for a lubricant coat film in an image bearing member where a lubricant coat film is formed on a photosensitive layer surface which bears a latent image. It is noteworthy that a surface protective layer having a large number of depressions and protrusions on its surface is provided between the photosensitive layer and the lubricant coat film. As described in the paragraph [0068] of JP-A No. 2007-121908, by forming the depressions and protrusions as the surface configuration of a photoconductor, it is expected that the adhesive force of the lubricant coat film will be increased and that the amount of shaving off of the lubricant caused by a cleaning blade will be reduced.
However, JP-A No. 2007-121908 does not explain the configuration of the depressions and protrusions of the photoconductor surface, except for the expression 10 nm<Rz<5,000 nm in measurement with the measurement length of 10 μm, and thus it is still unclear specifically what depressions and protrusions should be provided on the photoconductor, although it is understandable that if a photoconductor has a smooth surface, it causes difficulties. For example, even if the value Rz (surface roughness) is constant, the value Rz is calculated as an average value, and thus a photoconductor surface can have a variety of surface configurations. Therefore, it cannot be said that the description is practically defined. In addition, the improvement in adhesive force between the lubricant and the photoconductor may bring about another problem with removability of degraded lubricant.
JP-A No. 57-94772 proposes a surface treatment method for an organic electrophotographic photoconductor, in which a lubricant made of a low-surface-energy substance is directly applied to the organic electrophotographic photoconductor surface or the lubricant is incorporated into a dry developer and indirectly applied to the organic electrophotographic photoconductor surface, thereby improving cleaning effects, wherein the organic electrophotographic photoconductor surface is treated with a metal wire (13 μm to 20 μm in diameter) made of a material and selected from tungsten, molybdenum, nickel and stainless steel to form in the photoconductor surface a large number of grooves in the form of thin lines.
In this proposed technique, the method of forming the grooves in the photoconductor surface causes scratches to form in the photoconductor surface by means of the metal wire, and treatment of powder generated by the cutting, and the formation of the grooves are arbitrarily carried out depending on the situation. For this reason, it is considered difficult to produce photoconductors having the same surface configuration. Moreover, since the grooves in the form of thin lines have a pitch of 4 μm to 9 μm and the groove pattern is similar to the concavo-convex pattern seen on the surface of ground glass, the lubricant will be brought into line-contact with the photoconductor unless the lubricant is greatly reduced in particle diameter, and this may lead to insufficient adhesion of the lubricant to the photoconductor. In other words, the photoconductor surface may become a surface to which the lubricant is not easily attached.
JP-A No. 57-94772 discloses a technique wherein minute depressions present in the surface of a photoconductive material layer of a photoconductor in which a photoconductive material is attached to and formed on a substrate are filled with a selected material so as to flatten the photoconductor surface.
When the depressions (grooves) are provided in the photoconductor surface and a lubricant can be supplied to and removed from the depressions, it is, apparently, expected that high lubricating properties of the photoconductor will be sustained. JP-A No. 57-94772 proposes to embed, in the depressions, a material having electric resistance that is as high as that of the photoconductor surface in which the depressions are to be formed, for the purpose of preventing formation of abnormal images attributable to the depressions. From a different viewpoint, the following can be said: when depressions are formed in a material having high resistance, such as a solid solution containing a charge transporting material and polycarbonate generally used in a charge transporting layer, there are great effects caused by the depressions in terms of electrostatic properties and thus abnormal images are easily formed. Therefore, countermeasures against this problem are needed. Accordingly, attempts have been keenly made to strengthen the surfaces of organic photoconductors for the purpose of preventing formation of such abnormal images.
The techniques described in JP-A Nos. 2007-121908 and 57-94772 are conventional techniques relating to a combination of a specific surface configuration of a photoconductor and a lubricant. It is known that a lubricant is degraded by a charging step in an electrophotographic process. When the degraded lubricant unnecessarily remains on the photoconductor surface, there are problems caused such as an increase in driving torque of the photoconductor and breakdowns of members (a cleaning blade and the like) which rub against the photoconductor. The lubricant needs to be circulated such that it is appropriately supplied to and removed from the photoconductor surface; in related art, however, the techniques for enhancing the circulation efficiency are still insufficient.
JP-A No. 2006-11047 discloses an electrophotographic photoconductor in which countless linear scratches that intersect each other are uniformly formed on an reinforced surface of the photoconductor. The invention disclosed in JP-A No. 2006-11047 is originally a technique of performing surface treatment for the purpose of preventing formation of abnormal images, and so it cannot be directly applied to improvement in the applicability of a lubricant, which is a different purpose. Specifically, this is because the average width of the linear scratches is 10 μm or less and thus it is feared that the fixability of the lubricant could be insufficient, similarly to the case of JP-A No. 57-94772. In relation to the fact that (even though the photoconductor is provided with the reinforced surface) the photoconductor is more or less used in a process associated with abrasion, in the case where the photoconductor is utilized in an electrophotographic apparatus, JP-A No. 2006-11047 proposes that the photoconductor be used together with a surface shape reproducing device provided in the electrophotographic apparatus.
Methods for providing a photoconductor surface with a specific patterned shape have been known in the art for a long time. For example, JP-A No. 51-129237 discloses a method of partially irradiating the surface of a resin coating film (ionizing radiation curable resin coating film) with ionizing radiation through a wire gauze, a metal plate with countless small holes formed therein, a metal plate perforated in a patterned manner, or a metal frame corresponding to an irradiation pattern. In this case, however, it is necessary to take the trouble to dissolve uncured portions. Moreover, equipment needs to be designed in a manner that prevents the ionizing radiation from entering masked portions.
JP-A 63-106757 proposes patterning of a photoconductor whose outermost surface is provided with a plurality of depressions (grooves) each having a depth of λ/4 or greater (λ denotes a coherent exposure wavelength). Suppression of the formation of moire-related abnormal images is intended by forming the depressions with a specific period. This proposal is only aimed at preventing moire and thus does not pay attention to variation in the depth of the grooves. Moreover, since the pattern is formed on polycarbonate (which is relatively easily abraded) and formed using a metal brush, this proposal is deficient in durability and patterning uniformity.
The techniques disclosed in JP-A Nos. 2006-11047, 51-129237 and 63-106757, which have been described above, are typical prior-art techniques for forming a specific shape on the photoconductor surface. In the case where the shape is formed by scratching the photoconductor surface, however, it is difficult to always form the same pattern. It is also difficult to form such a uniform pattern as enlarges areas separated from each another by the grooves.
JP-A No. 60-202446 proposes a method of forming a mosaic filter of three primary colors on a photoconductor by ink jetting, as a production method of a one shot color electrophotographic photoconductor. It is explained that the mosaic pattern is a line pattern having a width of 100 μm and a thickness of 1 μm. Further, it proposes a special coating theory using an inkjet method. In the case where coating is performed by ink jetting, droplets of an ink may not be able to be ejected from an inkjet head or the droplets may be repelled by a base, unless the base and the ink are appropriately adjusted. It is not easy to realize coating by an inkjet method. Especially when the base is formed of a sparingly soluble curable resin and a silicone oil is attached to the surface thereof, the droplets are significantly repelled by the silicone oil.
JP-A No. 2006-337687 discloses a method of forming a protective layer, using an inkjet method. This method shows that the employment of coating by an inkjet method achieves an increase in the pot life of a thermosetting coating material, which starts reacting once two liquids are mixed together, and high production efficiency. A satisfactory protective layer can be formed with a thin film, and the coating material can be prepared using monomer component(s) only, without using any polymer components; this enables the coating by an inkjet method.
Similarly to the technique disclosed in JP-A No. 2006-337687, JP-A No. 2008-299261 discloses a technique which uses an inkjet method in forming a curable resin film. The pot life of a coating material can be increased by jetting two liquids to be cured, from separate droplet ejection heads. Also, by controlling the amounts of the two liquids ejected, utilizing an inkjet method, regulations can be realized such as provision of a bias to the ratio between the two liquids contained in the film. However, in order to stabilize the ejection of the liquids from the inkjet heads, the liquids often need ejecting for test purposes. For this reason, the coating efficiency is not necessarily high.
The techniques disclosed in JP-A Nos. 60-202446, 2006-337687 and 2008-299261, which have been described above, are typical related-art techniques related to film formation at the photoconductor surface by an inkjet method. These techniques have possibilities in the production of a surface layer of a photoconductor by an inkjet method; however, related art leaves some problems unsolved concerning the repellence of droplets ejected from inkjet head(s) and the ejection stability of the droplets themselves.
As described above, the related art cannot sufficiently enhance circulation of a lubricant supplied onto and removed from the surface of a photoconductor, and thus high durability of an electrophotographic photoconductor and cleaning stability of an electrophotographic apparatus have yet to be obtained.